SLAVIANSK, Ukraine - Pro-Russian separatists in Slaviansk in eastern Ukraine said on Friday Ukrainian forces had launched a "large-scale operation" to retake the town and one military helicopter had been shot down.

A Reuters photographer said he saw a military helicopter open fire on the outskirts of the town and a reporter heard gunfire.

In Kiev, an aide to Ukrainian Interior Minister Arsen Avakov said he could not comment.

"Until it's over, no one will say anything," he said.

Armed groups seeking union with Russia have seized a number of government buildings in towns in eastern Ukraine.